Many electronic systems include circuits, such as switching power converters or transformers that interface with a dimmer. The interfacing circuits deliver power to a load in accordance with the dimming level set by the dimmer. For example, in a lighting system, dimmers provide an input signal to a lighting system. The input signal represents a dimming level that causes the lighting system to adjust power delivered to a lamp, and, thus, depending on the dimming level, increase or decrease the brightness of the lamp. Many different types of dimmers exist. In general, dimmers generate an output signal in which a portion of an alternating current (“AC”) input signal is removed or zeroed out. For example, some analog-based dimmers utilize a triode for alternating current (“triac”) device to modulate a phase angle of each cycle of an alternating current supply voltage. This modulation of the phase angle of the supply voltage is also commonly referred to as “phase cutting” the supply voltage. Phase cutting the supply voltage reduces the average power supplied to a load, such as a lighting system, and thereby controls the energy provided to the load. A particular type of phase-cutting dimmer is known as a trailing-edge dimmer. A trailing-edge dimmer phase cuts from the end of an AC cycle, such that during the phase-cut angle, the dimmer is “off” and supplies no output voltage to its load, but is “on” before the phase-cut angle and in an ideal case passes a waveform proportional to its input voltage to its load.
FIG. 1 depicts a lighting system 100 that includes a trailing-edge, phase-cut dimmer 102 and a lamp 142. FIG. 2 depicts example voltage and current graphs associated with lighting system 100. Referring to FIGS. 1 and 2, lighting system 100 receives an AC supply voltage VSUPPLY from voltage supply 104. The supply voltage VSUPPLY, indicated by voltage waveform 200, is, for example, a nominally 60 Hz/110 V line voltage in the United States of America or a nominally 50 Hz/220 V line voltage in Europe. Trailing edge dimmer 102 phase cuts trailing edges, such as trailing edges 202 and 204, of each half cycle of supply voltage VSUPPLY. Since each half cycle of supply voltage VSUPPLY is 180 degrees of the supply voltage VSUPPLY, the trailing edge dimmer 102 phase cuts the supply voltage VSUPPLY at an angle greater than 0 degrees and less than 180 degrees. The phase cut, input voltage VΦ_DIM to lamp 142 represents a dimming level that causes the lighting system 100 to adjust power delivered to lamp 142, and, thus, depending on the dimming level, increase or decrease the brightness of lamp 142.
Dimmer 102 includes a timer controller 110 that generates dimmer control signal DCS to control a duty cycle of switch 112. The duty cycle of switch 112 is a pulse width (e.g., times t1−t0) divided by a period of the dimmer control signal (e.g., times t3−t0) for each cycle of the dimmer control signal DCS. Timer controller 110 converts a desired dimming level into the duty cycle for switch 112. The duty cycle of the dimmer control signal DCS is increased for lower dimming levels (i.e., higher brightness for lamp 142) and decreased for higher dimming levels. During a pulse (e.g., pulse 206 and pulse 208) of the dimmer control signal DCS, switch 112 conducts (i.e., is “on”), and dimmer 102 enters a low resistance state. In the low resistance state of dimmer 102, the resistance of switch 112 is, for example, less than or equal to 10 ohms. During the low resistance state of switch 112, the phase cut, input voltage VΦ_DIM tracks the input supply voltage VSUPPLY and dimmer 102 transfers a dimmer current iDIM to lamp 142.
When timer controller 110 causes the pulse 206 of dimmer control signal DCS to end, dimmer control signal DCS turns switch 112 off, which causes dimmer 102 to enter a high resistance state (i.e., turns off). In the high resistance state of dimmer 102, the resistance of switch 112 is, for example, greater than 1 kiloohm. Dimmer 102 includes a capacitor 114, which charges to the supply voltage VSUPPLY during each pulse of the dimmer control signal DCS. In both the high and low resistance states of dimmer 102, the capacitor 114 remains connected across switch 112. When switch 112 is off and dimmer 102 enters the high resistance state, the voltage VC across capacitor 114 increases (e.g., between times t1 and t2 and between times t4 and t5). The rate of increase is a function of the amount of capacitance C of capacitor 114 and the input impedance of lamp 142. If effective input resistance of lamp 142 is low enough, it permits a high enough value of the dimmer current iDIM to allow the phase cut, input voltage VΦ_DIM to decay to a zero crossing (e.g., at times t2 and t5) before the next pulse of the dimmer control signal DCS.
Dimming a light source with dimmers saves energy when operating a light source and also allows a user to adjust the intensity of the light source to a desired level. However, conventional dimmers, such as a trailing-edge dimmer, that are designed for use with resistive loads, such as incandescent light bulbs, often do not perform well when supplying a raw, phase modulated signal to a reactive load such as a power converter or transformer, as is discussed in greater detail below.
FIG. 3 depicts a lighting system 101 that includes a trailing-edge, phase-cut dimmer 102, an electronic transformer 122, and a lamp 142. Such a system may be used, for example, to transform a high voltage (e.g., 110V, 220 V) to a low voltage (e.g., 12 V) for use with a halogen lamp (e.g., an MR16 halogen lamp). FIG. 4 depicts example voltage graphs associated with lighting system 101.
As is known in the art, electronic transformers operate on a principle of self-resonant circuitry. Referring to FIGS. 3 and 4, when dimmer 102 is used in connection with transformer 122 and a low-power lamp 142, the low current draw of lamp 142 may be insufficient to allow electronic transformer 122 to reliably self-oscillate.
To further illustrate, electronic transformer 122 may receive the dimmer output voltage VΦ_DIM at its input where it is rectified by a full-bridge rectifier formed by diodes 124. As voltage VΦ_DIM increases in magnitude, voltage on capacitor 126 may increase to a point where diac 128 will turn on, thus also turning on transistor 129. Once transistor 129 is on, capacitor 126 may be discharged and oscillation will start due to the self-resonance of switching transformer 130, which includes a primary winding (T2a) and two secondary windings (T2b and T2c). Accordingly, as depicted in FIG. 4, an oscillating output voltage Vs 400 will be formed on the secondary winding of transformer 132 and delivered to lamp 142 while dimmer 102 is on, bounded by an AC voltage level proportional to VΦ_DIM.
However, as mentioned above, many electronic transformers will not function properly with low-current loads. With a light load, there may be insufficient current through the primary winding of switching transformer 130 to sustain oscillation. For legacy applications, such as where lamp 142 is a 35-watt halogen bulb, lamp 142 may draw sufficient current to allow transformer 122 to sustain oscillation. However, should a lower-power lamp be used, such as a six-watt light-emitting diode (LED) bulb, the current drawn by lamp 142 may be insufficient to sustain oscillation in transformer 122, which may lead to unreliable effects, such as visible flicker and a reduction in total light output below the level indicated by the dimmer.
In addition, traditional approaches for providing compatibility between a low-power lamp and the power infrastructure to which it is coupled have numerous shortcomings. For example, methods and systems for providing compatibility between a low-power lamp and the power infrastructure to which it is coupled are described in U.S. Patent Publication No. 2014/0009078 entitled “Systems and Methods for Low-Power Lamp Compatibility with a Trailing-Edge Dimmer and an Electronic Transformer,” filed on Mar. 13, 2013 and U.S. Patent Publication No. 2014/0028214 entitled “Systems and Methods for Low-Power Lamp Compatibility with a Trailing-Edge Dimmer and an Electronic Transformer” filed on Sep. 27, 2013. U.S. Patent Publication No. 2014/0009078 discloses systems and methods for predicting based on an electronic transformer secondary signal an estimated occurrence of a high-resistance state of a trailing-edge dimmer, wherein the high-resistance state occurs when the trailing-edge dimmer begins phase-cutting an alternating current voltage signal and operating the load in a high-current mode for a period of time immediately prior to the estimated occurrence of the high-resistance state. U.S. Patent Publication No. 2014/0009078 discloses systems and methods for (i) predicting based on an electronic transformer secondary signal an estimated occurrence of a high-resistance state of a trailing-edge dimmer coupled to a primary winding of an electronic transformer, wherein the high-resistance state occurs when the trailing-edge dimmer begins phase-cutting an alternating current voltage signal; (ii) operating a power converter in a trailing-edge exposure mode for a first period of time immediately prior to the estimated occurrence of the high-resistance state, such that the power converter is enabled to transfer energy from the secondary winding to the load during the trailing-edge exposure mode; and (iii) operating the power converter in a power mode for a second period of time prior to and non-contiguous with the first period of time, such that the power converter is enabled to transfer energy from the secondary winding to the load during the power mode. A disadvantage of these approaches are that when such approaches are employed in a single-stage power converter, charge delivery from the electronic transformer to the load may not be consistent, which may result in undesirable effects, including flicker.